Fuel cells electrochemically oxidize fuel such as hydrogen and methanol within the fuel cells. Thereby, fuel cells directly convert chemical energy of the fuel into electric energy and extract it. For this, fuel cells receive attention as a clean electric energy supply source. Particularly, polymer electrolyte fuel cells are expected as an alternative power source for automobiles, home co-generation systems, portable generators, and the like because of its operation at a lower temperature as compared to other fuel cells.
Such a polymer electrolyte fuel cell includes at least a membrane electrode assembly in which a gas diffusion electrode, which is produced by laminating an electrode catalyst layer (anode catalyst layer, cathode catalyst layer) and a gas diffusion layer, is bonded to both surfaces of a proton exchange membrane. Here, the proton exchange membrane is a polymer electrolyte membrane comprising a composition having a strong acidic group such as a sulfonic acid group and a carboxylic acid group in the polymer chain and having properties to selectively transmit protons. As an example of such a composition used for the proton exchange membrane, perfluoro proton composition such as Nafion (registered trademark, made by E. I. du Pont de Nemours and Company) which has a high chemical stability is exemplified and is preferably used.
During operation of a fuel cell, a fuel (for example, hydrogen) is fed to an anode gas diffusion electrode, and an oxidizing agent (for example, oxygen or air) is fed to a cathode gas diffusion electrode respectively. The two electrodes are connected via an external circuit to attain the operation of the fuel cell. Specifically, when hydrogen is used as a fuel, hydrogen is oxidized on an anode catalyst in the anode catalyst layer and then protons are produced. After passing through a proton-conductive polymer in the anode catalyst layer, the protons move inside of the proton exchange membrane and pass through a proton-conductive polymer in the cathode catalyst layer, and then reach a cathode catalyst in the cathode catalyst layer. Meanwhile, the electrons which are produced by oxidation of hydrogen at the same time of protons pass through the external circuit, and reach the cathode gas diffusion electrode. On the cathode catalyst in the cathode electrode layer, the protons react with oxygen of the oxidizing agent and then water is generated. At this time, electric energy is extracted.
At this time, the proton exchange membrane also needs to serve as a gas barrier by reducing gas permeability. A proton exchange membrane having high gas permeability causes cross leakage, that is, leakage of hydrogen on the anode side to the cathode side and leakage of oxygen on the cathode side to the anode side. Occurrence of the cross leakage causes the so-called chemical short, preventing extraction of good voltage. Moreover, the occurrence of the cross leakage causes the problem that hydrogen on the anode side reacts with oxygen on the cathode side to generate hydrogen peroxide, which chemically deteriorates the proton exchange membrane. In order to solve these problems, proton exchange membranes have been proposed in which chemical durability is improved by an ion exchange resin composited with an additive having an effect of suppressing production of hydrogen peroxide such as polybenzimidazole and polyphenylene sulfide (see Patent Literatures 1 and 2).
Meanwhile, from the viewpoint of reducing internal resistance of the fuel cell and further increasing the output of the fuel cell, reduction in the thickness of the proton exchange membrane serving as the electrolyte has been examined. However, the effect as the gas barrier is reduced by reduction in the thickness of the proton exchange membrane and then the problem of cross leakage becomes more serious. Further, since mechanical strength of the proton exchange membrane itself is reduced by reduction in the thickness of the proton exchange membrane, leading to physical problems. For example, the proton exchange membrane is difficult to handle in production of the membrane electrode assembly or assemble of the fuel cell and the proton exchange membrane is broken when the proton exchange membrane absorbs water generated on the cathode side to change the size of the membrane.
Then, in order to solve these problems, proton exchange membranes have been proposed in which a porous membrane is filled with an ion exchange resin (see Patent Literatures 3 to 5).
For recent demands for a longer operation time of the fuel cell, a higher temperature, and lower humidity, a membrane having both physical durability and chemical durability at the same time has been proposed in which a porous membrane is filled with an ion exchange resin composited with an additive such as polybenzimidazole and polyphenylene sulfide (see Patent Literature 6).